1. Field of the Invention
The present invention relates to an acoustic backing composition, an ultrasonic probe comprising an acoustic backing member formed of the acoustic backing composition and serving to transmit-receive an ultrasonic signal to and from, for example, an object, and an ultrasonic diagnostic apparatus comprising the ultrasonic probe.
2. Description of the Related Art
A medical ultrasonic diagnostic apparatus or ultrasonic image inspecting apparatus transmits an ultrasonic signal to an object and receives an echo signal from within the object so as to form an image of the inside of the object. An array type ultrasonic probe capable of transmitting and receiving an ultrasonic signal is used mainly in these ultrasonic diagnostic and ultrasonic imaging apparatuses.
The ultrasonic probe comprises an acoustic lens and a piezoelectric element. In performing a medical diagnosis by using the ultrasonic probe, the piezoelectric element is driven under the state that the ultrasonic probe on the side of the acoustic lens contacts against an object so as to transmit an ultrasonic signal from the front surface of the piezoelectric element into the object. The ultrasonic signal is converged at a prescribed position within the object by the electronic focus function produced in accordance with the drive timing of the piezoelectric element and by the focus function produced by the acoustic lens. In this case, it is possible to transmit the ultrasonic signal within a prescribed area within the object by controlling the drive timing of the piezoelectric element, and the echo signal is received from the object and processed in the ultrasonic probe so as to obtain an ultrasonic image (tomographic image) within the prescribed range noted above. The ultrasonic signal is also released to the back surface by the driving of the piezoelectric element. Therefore, an acoustic backing member is arranged on the back surface of the piezoelectric element absorb (attenuate) the ultrasonic signal transmitted to the back surface, thereby avoiding a detrimental effect that the normal ultrasonic signal is transmitted into the object together with the ultrasonic signal (echo signal) reflected from the back surface.
The conventional acoustic backing member comprises an epoxy resin used as a base resin and a powdery material loaded as a filler in the base resin. A high density powder such as a tungsten (W) powder, a lead (Pb) powder or a zinc oxide (ZnO) powder is used as the powdery material loaded in the base resin. The acoustic backing member has a density of about 2.0 g/cm3, a sound velocity of about 2,500 m/s, and an acoustic impedance of about 5 MRalys.
An acoustic backing member comprising a rubbery material used as a base resin such as a chloroprene rubber (CR), a butyl rubber or a urethane rubber and a powdery material having a high density such as W, Pb or ZnO, which is loaded as a filler in the base resin, is described in “Haifeng Wan et al., IEEE Transaction Ultrasonic Ferroelectrics and Frequency Control, vol. 48, No. 1, p. 78, 2001”. The acoustic backing member described in this publication has a density of about 3.0 g/cm3, an sound velocity of about 1,500 m/s and an acoustic impedance of about 5 MRalys.
Disclosed in Japanese Patents No. 3,420,951 and No. 3,420,954 are ultrasonic probes. One of these ultrasonic probes is constructed such that a sheet of a material having a high heat conductivity such as aluminum nitride, boron nitride, copper or carbon is arranged between the piezoelectric element and the acoustic backing member. The other ultrasonic probe comprises an acoustic backing member containing aluminum nitride, silicon carbide or copper as the filler. The ultrasonic probes disclosed in these patent documents permit efficiently releasing heat to the back surface of the piezoelectric element.
Disclosed in Japanese Patent Disclosure (Kokai) No. 60-68832 is an ultrasonic probe including a back surface layer exhibiting anisotropic acoustic characteristics. It is taught that the ultrasonic probe comprises metal fibers arranged on a synthetic resin such as an epoxy resin or an acrylic resin or on a compound material formed of rubber and that these metal fibers are aligned in the direction equal to the vibrating direction of a piezoelectric oscillator.
Further, an acoustic backing member formed of a preform and a matrix material is disclosed in Japanese Patent Disclosure No. 9-127955 (U.S. Pat. No. 5,648,941). It is taught that the preform denotes a linear fiber texture, a planar fiber texture such as a synthetic resin mesh sheet or a three dimensional fiber texture. It is also taught that the matrix material is used rubber and/or epoxy resin.
However, the acoustic backing member disclosed in each of the publications exemplified above gives rise to problems as pointed out below.
In preparing the ultrasonic probe, a piezoelectric element is bonded to an acoustic backing member, followed by bonding an acoustic matching layer to the piezoelectric element. Then, a dicing process is applied from the acoustic matching layer toward the acoustic backing member so as to divide the acoustic matching layer and the piezoelectric element into a plurality of arrayed sections, thereby forming a plurality of channels. Further, an acoustic lens is mounted to the acoustic matching layer for each channel. During the dicing process, grooves conforming with the diced portions are formed in the acoustic backing member. It is important to decrease the defective article ratio of the channels in order to improve the sensitivity in the ultrasonic probe of the particular construction. Also, in the ultrasonic diagnostic apparatus having an ultrasonic probe incorporated therein, it is important to decrease the defective article ratio of the channels in terms of the quality of the tomographic image. To be more specific, if the mechanical strength of the region between the adjacent grooves formed in the acoustic backing member is insufficient, the piezoelectric element included in the channel formed on the grooves is caused to collapse together with the acoustic backing member so as to make it impossible to use the channel.
The acoustic backing member described in each of the publications exemplified above comprises a base resin such as an epoxy resin or rubber such as a chloroprene rubber, a butyl rubber or an urethane rubber, and various fillers loaded in the base resin. The acoustic backing member of the particular construction is brittle, with the result that rupture or peeling is generated between the base resin and the filler by the stress during the dicing process. The rupture or peeling causes the acoustic backing member to be folded in the region between the adjacent grooves, or causes the peeling between the acoustic backing member and the piezoelectric element so as to bring about a defective channel. Particularly, where the dicing process is applied from the acoustic matching layer toward the acoustic backing member at a pitch of 50 to 200 μm in an attempt to reduce the channel size, to miniaturize the ultrasonic probe and to increase the density of the arrays, the folding of the acoustic backing member between the adjacent grooves and the peeling between the acoustic backing member and the piezoelectric oscillator are rendered more prominent by a large stress.
The peeling between the acoustic backing member and the piezoelectric oscillator can be improved to some extent by using an epoxy resin adhesive cured at a high temperature (120° C. or more) for bonding the acoustic backing member and the piezoelectric element. It should be noted, however, that, where a chloroprene rubber, a butyl rubber or a urethane rubber is used as the base material of the acoustic backing member, the acoustic backing member is deformed or denatured at the bonding temperature so as to render the bonding strength between the acoustic backing member and piezoelectric element insufficient after the bonding.
Also, in the acoustic backing member using a chloroprene rubber, a butyl rubber or a urethane rubber as the base resin, the performance of attenuating the ultrasonic wave is low. To be more specific, it is difficult to attenuate sufficiently the ultrasonic wave radiated from the piezoelectric element toward the acoustic backing member on the back surface. In order to allow the particular acoustic backing member to attenuate sufficiently the ultrasonic wave, it is necessary to increase the thickness of the acoustic backing member. However, if the thickness of the acoustic backing member is increased, it is difficult to decrease the weight and the heat dissipating properties of the ultrasonic probe.